This disclosure relates to inspection equipment and is applicable to, for example, defect inspection equipment using a mirror electron microscope.
When semiconductor devices are manufactured, fine circuits are formed over mirror-polished surfaces of wafers made of, for example, silicon (Si) or silicon carbide (SiC). When there are foreign objects, flaws, or crystal defects over wafer surfaces, they may cause various problems. For example, during a circuit pattern formation process, circuit pattern defects or material degradation may occur consequently to make manufactured devices inoperative, to make desired electrical characteristics unavailable, or to cause degradation of the operational reliability of manufactured devices. Such problems prevent devices from being finished as products.
For wafer inspection to be performed prior to a circuit pattern forming process, they have been using a technique (optical scattering inspection technique) in which light of a wavelength in a range from visible to ultraviolet (hereinafter referred to simply as “light”) is emitted to irradiate a wafer surface and in which light scattered at the wafer surface is detected and inspection equipment making use of an optical microscope technique, for example, a differential interference microscope. With the progress of semiconductor device miniaturization, however, it has become necessary to control foreign objects which, being so fine, cannot, generate, when irradiated with light, detectable scattered light intensities. Also, crystal defects which cannot be imaged using an optical microscope technique affect the reliability of semiconductor devices. Thus, existing inspection techniques using light can cause hindrance to wafer quality control.
An electron microscope technique using an electron beam can be used to detect foreign objects and defects which cannot be detected using light. An electron microscope has an extremely high spatial resolution and can easily image foreign objects sized smaller than 20 nanometers with remarkably low light scattering intensities. An electron beam is formed of charged particles, so that it can detect crystal defects which cannot be detected using light by making use of electrical properties of crystal defects. However, while an electron microscope can capture an image in a practical amount of time when observing a small micron-sized view field, it takes a great amount of time to observe the whole surface of a wafer to be a semiconductor substrate. For example, to inspect the whole surface of a silicon wafer measuring 100 mm in diameter with a resolution of about 10 nm, a scanning electron microscope requires about six days according to estimation based on typical conditions.
A new faster inspection method in which an electron beam is used is disclosed in Japanese Unexamined Patent Application Publication No. Hei 11 (1999)-108864. In the technique, a negative potential close to the acceleration. voltage applied to an irradiating electron beam is applied to the surface of a wafer to be inspected, thereby, causing an electron beam emitted toward a whole inspection view field over the wafer surface to turn back near the wafer surface. The turned-back electrons pass through an electron lens to form an image. Such turned-back electrons will hereinafter be referred to as “mirror electrons.” The technique disclosed in Japanese Unexamined Patent Application Publication No. Hei 11 (1999)-108864 is an inspection technique which uses a mirror electron microscope for making mirror electrons from an image.